2 possibilities :
either the NSS has detached the IMSI of that subscriber, because the perioic location update (T3212) wasn’t performed before the expiry of another timer that controls the IMSI detach on the NSS side.
or the cell in which the subscriber is doesn’t allow IMSI ATTACH (it is a cell parameter, ATT = ENABLE). Therefore when the subscriber switches on the mobile, the mobile is still not attach to the network. Only after it originates its first call, his IMSI will be attached to the network. But before his 1st call, the IMSI is not attached, so he can’t receive a call.